The use of blow-molding as a method for manufacturing various sorts of articles is well known. Typically, this process involves the use of a mold consisting of two separate halves or portions having cavities of particularly desired shapes and sizes. Usually, the manufacturer extrudes a large-diameter, sealed tube of molten material (usually plastic or polymer; commonly referred to as a “parison”), places the tube between the mold halves, and closes the mold around the tube. Fluid pressure is then introduced into the tube, forcing the molten tube against the walls of the cavities, causing the tube to conform to the shape of the mold. The pressure is maintained until the molten material cools and solidifies. The pressure is then released, the mold halves are pulled apart, and the hardened article is ejected.
The above-described blow-molding process lends itself to the fabrication of tube-like structures. To accomplish this, a cylindrical mold is used and the parison is inserted and inflated within the mold to create a hollow cylinder. The resulting blow-molded tubes are strong, lightweight, inexpensive to manufacture, durable, and reasonably aesthetically pleasing. A large proportion of blow-molded tubes manufactured are designed and used for packaging (i.e. a blow-molded tube being used as a canister to package a product). However, they can also be used as a light or medium duty modular structural element or frame component for semi-permanent structures. For example, blow-molded tubes can be used as a structural backbone for exterior structures such as lawn ornaments, mailboxes, toys, and modular furniture.
When used as a modular structural element, strength and ease of assembly are the primary design objectives. In addition to the tubes themselves being relatively strong, the coupling between tubes must also be strong. This is particularly true if the tube or tubes will be used as a load bearing element.
Such design objectives are difficult to accomplish given the blow-molding process described above. Although it is relatively easy to create a tube by blow-molding, integrating a coupling system into the blow-molded tube is not as simple. One reason is that blow-molding only allows detailed features to be molded on the outer surface of the tube. Manufacturers have previously used separate pieces to connect multiple tubes. This is disadvantageous due to added complexity and cost, the risk of component loss, diminished aesthetics, and potential choking hazards.
As a result of these manufacturing limitations, the integrated coupling systems found on today's blow-molded tubes leave much to be desired in terms of load bearing ability, coupling permanence, and stability of the coupling. Unfortunately, this has prevented the use of coupled blow-molded tubes in applications where they would otherwise be ideally suited due to their low cost and strength to weight ratio. Therefore, there remains a need in the art for a streamlined, elegant, integrated coupling system for blow-molded tubes that is permanent, stable, and can support a heavy load.